Landing strings are used in the oil and gas industry for through-riser deployment of equipment, such as completion architecture, well testing equipment, intervention tooling and the like into a subsea well from a surface vessel. When in a deployed configuration the landing string extends between the surface vessel and the wellhead, for example a wellhead Blow Out Preventer (BOP). While deployed the landing string provides many functions, including permitting the safe deployment of wireline or coiled tubing equipment through the landing string and into the well, providing the necessary primary well control barriers and permitting emergency disconnect while isolating both the well and landing string.
Well control and isolation in the event of an emergency disconnect is provided by a suite of valves which are located at a lower end of the landing string, normally positioned inside the central bore of the BOP. The BOP therefore restricts the maximum size of such valves. The valve suite includes a lower valve assembly called the subsea test tree (SSTT) which provides a safety barrier to contain well pressure, and an upper valve assembly called the retainer valve which isolates the landing string contents and can be used to vent trapped pressure from between the retainer valve and SSTT. Typically, the valves within a landing string provide a shear and seal capability, such that any objects present in the landing string, such as wireline, will be severed, allowing a seal to then be established.
The landing string also typically includes features allowing interaction with a BOP or wellhead architecture. For example, a shear sub component may extend between the retainer valve and SSTT which is capable of being sheared by the BOP if required. Also, one or more slick joints may be provided to allow sealing engagement with BOP pipe rams. Further, a lowermost end of a landing string typically includes a tubing hanger arrangement which mates with a wellhead tubing hanger assembly.
Many landing string designs operate under certain safety protocols, often dictated by industry standards. For example, in some instances valves, such as a retainer valve, may be designed to operate under a fail-close protocol, in which the valves will automatically close in the event of a loss of control, such as a loss in hydraulic power. In some instances this might be overly cautious, in that certain valve control failures may not necessarily present a real risk to loss of well control, for example where other well control barriers are fully intact and operational, where loss in control is temporary and/or intentional and the like. In circumstances where an object, such as wireline is present at the time of failure, a fail-close valve may unnecessarily sever the wireline, dropping any associated tooling or equipment into the wellbore, requiring time-consuming fishing operations to recover.
In other instances valves, such as a retainer valve, may be designed to operate under a fail-as-is protocol, in which the valve remains in position in the event of loss of control. While this might avoid severing an object such as wireline, this does present other issues such as where a genuine emergency situation arises in which a full closure of the valve would be preferred.
Furthermore, landing strings are often used to accommodate flow back from the well to a surface vessel, for example during well testing, clean-up and the like. Accordingly, the entire length of the landing string could potentially contain well fluids under pressure in the event of an emergency disconnect situation. In such circumstances it is the purpose of the retainer valve to contain the fluids within the landing string upon disconnect. Although this is particularly important in all wells, in gas wells the pressurised gas within the landing string will carry significant energy, and in the event of an emergency disconnect this could cause the upper landing string to eject upwardly through the vessel. As such, it is important for the retainer valve to react quickly, to ensure the landing string fluids are contained.